The present invention relates generally to methods and apparatus for converting a signal between the analog and digital domains, and more particularly, to methods and apparatus for converting a signal between the analog and digital domains using frequency interleaving.
Analog-to-digital (A/D) converters often employ time interleaving to reduce the required conversion speed using multiple analog-to-digital converters in parallel. Each parallel analog-to-digital converter operates at a proportionally slower speed to convert time-interleaved signal samples into the digital domain. Nonetheless, high-frequency RF signals must still be sampled accurately prior to each parallel analog-to-digital converter. Thus, despite the gains achieved with the reduced required conversion speed for each of the parallel analog-to-digital converters, the sampling accuracy requirement is not relaxed. It has been found that it is difficult to maintain a high conversion accuracy up to the Nyquist frequency (sampling frequency divided by two) when analog-to-digital conversion must be performed at 10 Gigasamples/second and beyond, even using the most exotic technologies and only low amplitude resolution, such as a four bit resolution.
As the operating speeds of applications increase, there is a corresponding need to increase the conversion speeds of the analog-to-digital converters used by such applications. Thus, the above-mentioned limitation caused by the sampling accuracy requirement becomes even more critical. Multi-Gigasamples/second analog-to-digital conversion, for example, has an increasingly important role in many applications, such as wideband optical communications, digital radio or high-speed oscilloscopes. A need therefore exists for methods and apparatus for converting a signal between the analog and digital domains that maintain a high conversion accuracy up to the Nyquist frequency. A further need exists for methods and apparatus for converting a signal between the analog and digital domains where both A/D conversion and signal sampling can be performed at much slower rates.
Generally, a method and apparatus are disclosed for converting a signal between the analog and digital domains using frequency interleaving. The disclosed analog-to-digital and digital-to-analog converters maintain a high conversion accuracy up to the Nyquist frequency. When converting a signal from the analog domain to a digital domain, for example, the input broadband signal is decomposed into N frequency bands that are separately sampled (quantized) before a Fourier transform is applied to convert the signal into the digital domain. Each of the frequency bands can be sampled in the corresponding narrow passband using narrow-band converters, such as passband Sigma-Delta converters, or can be returned to baseband prior to sampling. The various analog samples are then converted to the digital domain using an inverse Fourier transform, or another combining technique. In this manner, sampling and analog-to-digital conversion are both performed at a speed that is N times slower than the input frequency.
The present invention thus employs parallel analog-to-digital conversion of frequency-domain samples of the input signal. The disclosed frequency interleaving technique decomposes the input signal into frequency bands that are digitized separately at a slower rate. Time-domain samples are obtained by performing an inverse Fourier transform of the quantized frequency-domain samples. Analog-to-digital conversion and signal sampling are performed at a much lower rate, thereby maintaining a high conversion accuracy up to the Nyquist frequency and beyond. In addition, a calibration scheme is disclosed to correct for phase and gain mismatches.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.